Method for controlling legionella in cooling towers

ABSTRACT

A method of controlling  Legionella  bacteria in cooling water systems is described. The method involves the heating of recirculating cooling water to temperatures of 70° C. for sufficient time to kill the bacteria. The method functions best in combination with chemical bactericides.

TECHNICAL FIELD

This invention relates to methods for controlling bacteria, inparticular Legionella, in cooling towers.

BACKGROUND OF THE INVENTION

A respiratory disease was first identified in 1976 as being caused by abacteria that was later named Legionella pneumophila. The disease wasnamed Legionnaires' disease. Since then the disease has occurred in manylocations throughout the world including Australia. A recent seriousoutbreak occurred at the Melbourne Aquarium in 2000 and deaths werereported from this outbreak. In this case cooling towers associated withthe facility were identified as the source of the bacteria.

Although previously healthy people may contract the disease, those mostat risk are patients undergoing treatment for cancer and otherconditions which may render them immunosuppressed. Others at riskinclude smokers, heavy drinkers and persons with underlying medicalconditions such as diabetes and chronic obstructive lung diseases. Themean age of those afflicted has been in the mid-fifties with males beingmore likely than females.

Transmission is believed to be by inhalation of contaminated aerosols.Infection is not believed to be caused by drinking Legionellacontaminated water.

L. pneumophila, one of more than twenty currently recognised species inthe genus Legionella, accounts for approximately 75 percent of the casesreported in Australia in the State of Victoria. There are currentlyfourteen recognised serogroups of L. pneumophila. Infection with otherspecies, namely L. bozemanii and L longbeachae, has also occurred inVictoria. Strictly speaking, Legionnaires' disease is the term used forthe disease caused by L. pneumophila, legionellosis refers to thedisease caused by any species of Legionella. In this specification theterm “Legionnaires' disease” will be used to refer to the pneumonic formof the disease caused by any species of Legionella.

Legionnaires' disease manifests as severe pneumonia, with patientspresenting with early symptoms of malaise, muscle pains, headache andfever with drenching sweats. Within the first week, a cough, usuallydry, develops. This occasionally is accompanied with bloodstainedsputum. The patients become increasingly short of breath and therespiratory symptoms progress to pneumonia, often culminating inrespiratory failure. There is usually a multi-system involvement, withmental confusion and delirium, diarrhoea, vomiting and renal failure.The disease usually has an incubation period of two to ten days. In thetreatment of Legionnaires' disease, erythromycin is usually the drug offirst choice, with the possible addition of rifampicin in non-respondingcases.

Bacteria in the genus Legionella are widely distributed and are naturalinhabitants of waters. They have been found in lakes, rivers, creeks andother bodies of water. Other bacteria and algae can provide nutrientsfor Legionella and may themselves cause health problems if aerosolscontaining such bacteria or algae are inhaled.

As set out previously, the route of human infection is considered to beby the inhalation of aerosols containing Legionella. Aerosols of fivemicrons diameter or less are particularly effective at penetrating tothe lower human respiratory tract. Several of the bacteria in aerosolsare reported to be at a maximum at around 65 percent relative humidity.The risk of acquiring infection by Legionella increases with the numberof bacteria deeply inhaled and, therefore, with the proliferation ofthese bacteria in the water source and the extent to which they aredispersed in aerosols of suitable size.

Aerosols may be generated during the normal operation of cooling towers.

Cooling towers are provided in some air conditioning systems to removethe heat rejected by the chiller(s). They are also used for removingheat generated in many industrial processes including refrigeration ofair and liquids. In this specification we refer to these collectively asfluids. Typically, water is circulated from a basin via a heat exchangerto the top of the tower where it sprays or falls through a structurewhich is designed to create an extensive wetted surface area throughwhich air passes. The movement of the air causes evaporative cooling ofthe water. In this specification the term cooling tower includes coolingtowers of all configurations and includes heat rejecting equipment wherewater is circulated from a reservoir and sprays or falls over heatedmaterial. It involves air movement and evaporative cooling of the water.

In this specification the term chiller(s) means all equipment thatrejects heat to cooling water and cooling tower systems. In thisspecification the term cooling water includes condenser water.

In operation, the cooling water gains heat from chillers or other heatexchanger source, and in the process of being distributed over thetower, loses heat to the moving air through evaporative cooling andconvective and conductive heat exchange. The mode of airflow is usuallyforced in commercial systems but by forced and/or convection forindustrial plants.

In the operation of all cooling towers, water is lost throughevaporation, bleed-off and drift. Drift is the portion of thecirculating water entrained in the cooling tower discharge as very smalldroplets or aerosol. These are produced within the tower by waterimpacting on the tower fill and also by the water distribution system.The airflow may entrain the smaller droplets and carry them through thetower. To minimise the drift loss, eliminators are frequently locatedbefore the tower discharge. Water lost by the above processes isprovided by make up water which is usually delivered directly to thecooling tower reservoir or basin. The quantity of make up water isnormally one of the order of percent of the flow rate of therecirculating cooling water.

To maximise the effective life of the cooling tower and associatedequipment, it has been standard practice for decades to chemically treatthe circulating water. Corrosion inhibitors are used to minimise thecorrosion of metal surfaces. Surfactants, biocides and other chemicalsare used to control fouling due to scale, silt and microbial growths inorder to maintain efficient heat transfer at heat transfer surfaces,ensure free flow of water throughout the system, and prevent theproliferation of certain microorganisms which are responsible forsurface corrosion and degradation. Biocides must come into contact withthe microorganisms to ensure adequate control. Particulate matter,scale, debris, slimes and the presence of other microorganisms such asprotozoa have the potential to shield Legionella from biocides and thismay result in their persistence and proliferation if biocide levels fallor the Legionella are shielded. Since the increased awareness ofLegionnaires' disease potential from cooling towers, there has been anincreased use of biocides as a control method.

A major deficiency of the majority of biocides is the lack of a simpleon-site test to determine and continuously monitor their concentrationin cooling water. Consequently, initial biocidal concentration iscommonly determined by calculation based on the estimated water volumeof the system and the weight of biocide added. Further disadvantages ofthe reliance upon biocides are their high cost, handling hazards foroperators and adverse environmental effects, particularly in aerosolform and discharges to sewer. Additionally, there is the potentialproblem that bacteria may develop resistance necessitating the use ofnew, more potent biocides or combination of biocides. Furthermore, inpractice a common reaction to reduce the risk of Legionella is to dosemore frequently. The potential hazards from such high levels ofchemicals in aerosol form has not be assessed.

The approach to treating cooling towers to date has included compulsoryregistration, determining existing locations of all cooling towers,development of risk management plans, plus application of compulsorycleaning and chemical treatment standards and regimes.

Another approach has been to avoid the use of cooling towers.Alternative engineering solutions are available that use air cooledchillers or air cooled heat exchangers. However, the operation of thesechillers and air cooled heat exchangers involves significant additionalcosts because of high electricity or other energy usage and demand. Theadditional energy costs of air cooled chillers may be typically 30% overthe energy cost of a cooling tower system. It is desirable for systemsto be energy efficient and allow designers to select lowest life cyclecost alternatives. While the capital costs are not greatly different fora new installation, retrofit into existing systems would usually involvesubstantial additional costs.

A novel approach to disinfecting Legionella is disclosed in WO91/02935.In this patent application a method is described where the coolant waterin a collection tank that forms part of a cooling tower is heated to atemperature to kill the unwanted bacteria for a period of four hourseach month. Such a system does not appear to have been commercialisedand it is considered that such a treatment is not sufficientlyefficacious. In the system of this disclosure, one of the shortcomingsis that it does not address the problem of bacteria in the heatrejection equipment. The operation of the system also teaches thatmonthly treatment will control the bacteria levels between treatments.In practice it is believed that bacteria levels can significantlyincrease over periods of days or weeks. Additionally, the system of thisdisclosure is only suitable for use where the heat rejecting plant andcooling water system can be shut down during the time of treatment.Increasingly cooling towers are required to be available to supportchillers operating on an 24 hour basis.

Another novel approach is described in WO 90/02707. This prior artdiscloses a building cold water system including cooling tower make upwater where the make up water is pasteurised. However, this inventiondoes not address the issue of recirculated water from cooling towerswhich, as discussed previously, constitutes the vase majority of thewater in the system. The make up water normally represents onlyapproximately 1% of the recirculating cooling water. It is therecirculated water that is believed to contribute most to the undesiredbacteria. Only a very minor amount of undesired bacteria is believed tobe sourced from the inlet water.

It would be desirable to have a method of controlling Legionella incooling towers without being solely dependent on high levels ofbiocides.

SUMMARY OF THE INVENTION

In one form the invention provides a method of controlling Legionella byoperating a cooling water system for a fluid conditioning plant by atleast every 10 days heating the recirculating cooling water to anelevated temperature of at least 60° C. for a predetermined time,wherein heat sensitive heat rejecting equipment is isolated from theheated recirculating cooling water.

Preferably the elevated temperature at least 65° C.

Preferably the elevated temperature at least 70° C.

Preferably the elevated temperature is in the range 70 to 80° C.

Preferably the predetermined time is at least 5 seconds when theelevated temperature is at least 70° C.

Preferably the predetermined time is at least 30 minutes.

Preferably the predetermined time is at least 60 minutes.

Preferably the frequency of heating is at least once in every 24 hours

Preferably water in the isolated heat rejecting equipment is treatedwith preselected high doses of antibacterial chemicals, the doses beingselected such that when the heat rejecting equipment is not isolated,the dosage level in the overall cooling water system is in the desiredrange.

In an alternative form the invention provides a a method of controllingLegionella by operating a cooling water system for a fluid conditioningplant by heating the recirculating cooling water to an elevatedtemperature of at least 60° C. for a predetermined time by means of abypass heating system whereby a minor portion of the recirculatingcooling water is heated to the elevated temperature for a predeterminedperiod of time.

Preferably the minor portion of the recirculating cooling water is lessthan 5%, and more preferably less than 1%.

Preferably the first and second alternative forms of the invention areused in combination.

Preferably the method of controlling Legionella in a cooling watersystem for a fluid conditioning plant also includes maintaining thetemperature of the cooling water as close as practical to the ambientwet bulb temperature when the cooling water is not required for cooling,thereby minimising the growth of Legionella bacteria. This method may becombined with the first and/or second alternative forms of theinvention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of the process of the invention.

FIG. 2 is a schematic diagram of an alternative form of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The use of low temperatures for part of the cycle reduces themultiplication of the bacteria. The cycle involves the operation of thecooling water system without chiller heat release to consistently reducethe temperature of the cooling water when not required for heatrejection. The operation of the cooling water system at temperaturesclose to the ambient wet bulb temperature is carried out after theoperation of the chiller, that is when the chiller is not operating.This can be most readily achieved by the operation of cooling watersystem after chiller operation have ceased and at the colder times ofthe day.

The object of the above is to keep the cooling water as cool as ispractical when not required for cooling the chiller or other equipment.We have found these conditions reduce the rate of growth of Legionella.

The attainment of temperatures in excess of 60° C. involves a heatingsource. This is usually most cost effectively achieved by using thenormal hot water heater used to heat the building or process. Whereavailable, steam, desuperheaters and waste process heat or other heatsources may be utilised. A bypass system may be used to preventoverheating of heat rejecting equipment/chiller. Appropriate fail safesystems would be normally employed. For example, control systems may beused to avoid the overheating of refrigerant during the heated cycle. Toreduce the bacteria levels in the heat rejecting equipment/chillersduring the hating cycle we have found it beneficial to includeconventional biocides. These may be used at much higher levels thannormal as the volume of water in the heat rejecting equipment/chiller ismuch less than the total volume of recirculating cooling water. Levelstypically 5 or 10 times higher than normal may be used and these higherlevels are considerably more effective in killing the bacterial in theheat rejecting equipment/chiller.

Referring to FIG. 1, a cooling tower 1 is shown with a water basin 2 atthe base of the cooling tower 1. The cooling tower 1 has hightemperature tower fill material 3, air inlets 4, 5, drift eliminators 6,tower fans 7, and water distributors 8. The water distributors 8 are fedby a cooling water pipe 9. A water basin outlet 10 is connected by pipe11 and pump 12 and in turn to the cooling water pipe 9. Make up water ispiped 13 to the water basin 2. Heat rejection equipment 14 is located ina looped pipeway 15 with appropriate valve control 16, 17 to allowisolation of this equipment during the heating cycle. Heat 18 isintroduced into the recirculating water during the cleansing cycle. Theheat rejecting equipment 14 is able to be dosed with relatively highlevels of bactericides and with suitable recirculating equipment killsundesired bacteria in this equipment which is isolated from the heatcleansing equipment. During normal operation and in combination withthis heating cycle the recirculating cooling water may be cooled whenpossible to reduce the temperature of the water in the basin totemperatures less than 20° C. This reduces the multiplication of thebacteria. The operation of this system also controls the population ofother undesired bacteria and algae. The application of the technologydescribed above would be applicable to all fluid conditioning systemsthat do not operate on a 24 hour basis.

Alternatively, chillers with dedicated cooling towers may be utilised tofacilitate the heating cycle on one or more units while cooling iscontinued on the other(s). These systems could also be used on systemsthat operate on a 24 hour basis.

Effective control of Legionella was able to be achieved withconventional levels of biocides by heating the cooling water in acooling water tower for 5 minutes at 70° C. once in every 24 hours.Temperature monitoring and control were able to be achieved easily andthis included the capability of remote monitoring of the operation ofthe system.

Referring to FIG. 2, in this embodiment the heat rejection equipment 14is not isolated from the recirculating cooling water. The recirculatingcooling water is heated by a bypass heater and held in a holding tank 19and preheater heat exchanger 20. The preheater 20 is optional but ispreferred as it increases the efficiency of the operation of the system.The heating process may also be achieved by utilising waste heat, heatfrom desuperheaters or other sources. This system is suitable forintermittent and continuous use.

The processes of the invention would also usually involve automatedcontrols to action and control the low and high temperature cycles.Monitoring equipment to demonstrate that the cycles had been completedsatisfactorily would be included as part of a quality assurancerequirement.

The present invention also includes the operation of cooling watersystem such that the cooling and heating cycles are sequentiallycombined. While the present invention includes the control of Legionellawithout reliance on biocides, these materials may be used to augment theefficacy of the cooling water and/or heating system and these are ourpreferred systems. Although the present invention has been describedwith reference to cooling towers for air conditioning, it is alsoapplicable to other community, commercial and industrial situationswhere there is potential for aerosols with Legionella infected watersuppliers.

Since modifications within the spirit and scope of the invention may bereadily effected by persons skilled in the art, it is to be understoodthat the invention is not limited to the particular embodimentdescribed, by way of example, hereinabove.

1. A method of controlling Legionella by operating a cooling watersystem for a fluid conditioning plant comprising at least every 10 daysheating the recirculating cooling water to an elevated temperature of atleast 60° C. for a predetermined time, wherein heat sensitive heatrejecting equipment is isolated from the heated recirculating coolingwater.
 2. A method as defined in claim 1 wherein the elevatedtemperature is at least 65° C.
 3. A method as defined in claim 1 whereinthe elevated temperature is at least 70° C.
 4. A method as defined inclaim 1 wherein the elevated temperature is at least 70 to 80° C.
 5. Amethod as defined in claim 1 wherein the predetermined time is at least5 seconds when the temperature is at least 70° C.
 6. A method as definedin claim 1 wherein the predetermined time is at least 30 minutes.
 7. Amethod as defined in claim 1 wherein the predetermined time is at least60 minutes.
 8. A method as defined in claim 1 wherein the frequency ofheating is at least once in every 24 hours.
 9. A method of controllingLegionella in operating a cooling water system for a fluid conditioningplant comprising heating a minor portion of the recirculating coolingwater to an elevated temperature of at least 60° C. for a predeterminedtime by means of a bypass heating system.
 10. A method as defined inclaim 9 wherein the minor proportion of the cooling water is less than5%.
 11. A method as defined in claim 10 wherein the minor proportion ofthe cooling water is less than 1%.
 12. A method as defined in claim 9wherein the elevated temperature is at least 70° C.
 13. A method asdefined in claim 9 that further includes periodic heating according tothe method wherein heat sensitive heat rejecting equipment is isolatedfrom the heated recirculating cooling water.
 14. A method as defined inclaim 1 including maintaining the temperature of the cooling water asclose as practical to the ambient wet bulb temperature when the coolingwater is not required for cooling, thereby minimizing the growth ofLegionella bacteria.